The Transgenic Mouse/ES Cell Chimera (TgESC) Facility provides three basic services: production of transgenic (Tg) mice by zygote injection, production of embryonic stern (ES) cell chimeric mice and rederivation of mouse strains. For production of transgenic mice, users supply DNA constructs suitable for pronuclear injection into fertilized mouse eggs. Transgenic mice are produced in four different genetic backgrounds: B6D2/F1 hybrids, outbred Swiss Webster mice or inbred FVB/N and C57BL/6 mice. Potential founder animals are provided to the research investigator at weaning for genotyping. ES cell chimeras are produced by injectionof ES cells into C57BL/6 blastocysts or by aggregation with Swiss Webster morulae. Chimeras are provided to the research investigator at weaning for test breeding to determine germline transmission of the mutation. Aggregation chimeras between two strains of mice are also made by the facility and by special request aggregation chimeras can be made with tetraploid morulae to produce embryos that are 100% ES cell-derived. To enable investigators to house pathogen-free animals in the SPF animal quarters in Skirball, rederivation of mouse strains is also offered. In addition, the facility provides researchers with a wide array of services related to the production of transgenic and knockout ES cells and chimeric mice such as supply of superovulated egg donors, pseudopregnant females, training in mouse husbandry and consultation intransgenic/ES cell approaches and vector design. A new service of mouse sperm cryopreservation is under development to allow researchers at NYUSoM to preserve valuable strains of mice that are not being used at the time.